The present invention relates to a method for manufacturing a gas electron multiplier (GEM). The structure and the operation of a GEM are described in EP 0 948 803 B1, in which also a number of further references are given. FIG. 1 is a schematic diagram taken from EP 0 948 803 B1 showing the general structure and function of a GEM. In FIG. 1, a GEM 10 is located between a drift electrode DE and a collecting electrode CE. The GEM 10 consists of an insulator sheet 12 which is cladded with first and second metal layers 14, 16. In the GEM 10, a plurality of throughholes 18 are formed. The throughholes 18 typically have a diameter of 20 to 100 μm. The holes 18 are arranged in a matrix or array pattern with a pitch of typically 50 to 300 μm. A schematic view of the matrix of holes 18 is shown in FIG. 3, which has been taken from EP 0 948 803 B1 as well. The thickness of the insulating sheet 12 could be about 50 μm and the thickness of the first and second metal cladding layers 14 and 16 are typically about 5 μm thick.
Briefly, the function of GEM 10 of FIG. 1 is summarized as follows. A voltage is applied between the drift electrode DE and the collecting electrode CE. In addition, a voltage is applied between the first and second metal layers 14, 16 such that each of the holes 18 behaves like an electric dipole. The electric dipole is represented by an electric field vector {right arrow over (E)}′, which is superposed with the electric field {right arrow over (E)} between the drift electrode DE and GEM 10 and the electric field {right arrow over (E)}″ between the GEM 10 and the collecting electrode CE. The superposition of the three mentioned field components leads to the electrical field line structure schematically indicated in FIG. 1. As can be seen from FIG. 1, the holes 18 lead to a local condensation of the electrical field, or in other words a local electric field amplitude enhancement. The space between the drift electrode DE and the collecting electrode CE is filled with a gas. If a primary electron is generated somewhere between the drift electrode DE and the GEM 10, the electron drifts toward the GEM due to the electric field {right arrow over (E)}. In the hole 18, the electric field amplitude is locally enhanced such that an electron avalanche is formed from this primary electron, where the second metal layer 16 acts as a terminal interface for the electron avalanche. The formation of the electron avalanche from a primary electron is what makes GEM an “electron multiplier”. The electron avalanche is then attracted to the collecting electrode CE by the electric field, where it can be detected as a largely enhanced signal.
While FIGS. 1 and 3 only show a very small fraction of GEM 10, FIG. 2, which is also taken from EP 0 948 803 B1, shows a schematic view of the overall device. As can be seen from FIG. 2, the GEM 10 generally consists of an active area 20 in which the metal layers 14, 16 and the plurality of holes are formed. This active area 20 is surrounded by a frame 22, which is not metal-coated, but typically only consists of the insulating sheet 12. On frame 22, first and second electrodes 24 and 26 are formed on opposite sides thereof, which allow to apply the desired electrical potential to the first and second metal layers 14 and 16.
EP 0 948 803 B1 also discloses a method for manufacturing the GEM 10. According to said prior art method, two identical films or masks are imprinted with a desired pattern of holes and overlaid on each side of the metal cladded blank GEM which is previously coated with a light-sensitive resin. After exposure with ultraviolet light and development of the resin, the resin exposes only the portions of the metal layers 14, 16 corresponding to the holes to be formed. Then, the metal layers are etched simultaneously from both sides, such that holes are grown from both sides which meet in the middle to form the throughholes 18.
The prior art manufacturing method relies on the co-registering of the films or masks used for exposing the light-sensitive resin. A good coincidence of the patterns on both sides of the blank GEM can in fact be obtained if the active area 20, i.e. the area where the holes 18 are to be formed, is not too large, say 10×10 cm. However, recently there has been a demand for larger sized GEMs. When trying to manufacture bigger GEMs, the inventor found that difficulties arise with the prior art manufacturing method. In particular, for larger GEMs it turns out to be very difficult to ensure a proper co-registering of the patterns on both sides of the blank. As mentioned above, conventionally, a photomask had been directly placed on top of each of the first and second metal layers 14, 16 which were covered with a photoresist. While it is possible to print these masks with sufficient precision, it turned out that the film on which the masks were printed were not stable enough to guarantee a precise alignment of the pattern on both sides of the blank if the films are becoming larger such as to form a larger GEM. In particular, the films tend to slightly deform due to temperature and/or humidity, and given the very small size of the holes to be formed, this distortion is already enough to severely disturb the co-registering of the two patterns, which then leads to holes in which the center axes of the two halves formed from opposite sides are shifted by an unacceptable amount of 15 μm or more.
The inventor have also made attempts to circumvent these problems by using a mask material that is more stable. For example, attempts have been made to make such masks from glass. However, the results were not satisfactory. In particular, for the desired large mask sizes, the lack of planarity of the glass turned out to be a problem.
It is an object of the present invention to provide a method for manufacturing a GEM 10 that allows to manufacture high quality GEMs even at large sizes.